The contactless power transmission technology (also called wireless power transmission technology) is widely used in electronic charging field with its advantage of safety and convenience. There are two ways to achieve wireless power transmission: magnetic induction and magnetic resonance. Normally the magnetic resonance way is used. The magnetic-resonance wireless power transmission device mainly includes a transmitting portion and a receiving portion, both can achieve energy transmission according to the electromagnetic resonance principle.
Generally, the transmitting portion includes an inverter, an impedance matching circuit and a transmitting coil. The receiving portion includes a receiving coil, an impedance matching circuit, and a rectifying circuit (taking a full-bridge rectifier as an example) and a DC voltage conversion circuit (DC-DC converter).
As shown in FIG. 1, the transmitting portion of the contactless power transmission device only indicates a transmitting coil. The transmitting coil LS receives alternating current to generate alternating magnetic field with frequency of ω0, after the receiving coil Ld induces the alternating voltage Vsin(ω0) with frequency of wω0, through the rectification of full bridge rectifier circuit and filter capacitor C compensation, the alternating voltage Vsin is converted to DC voltage Vrect.
When coupling of receiving coil Ld and transmitting coil LS, the alternating voltage amplitude induced from the receiving coil is high; and when poor coupling, the alternating voltage amplitude induced from the receiving coil is low. Therefore, in order to induce the maximum amplitude of DC voltage Vrect at the receiving terminal, the resonant frequency of the capacitor C and the inductance L of the coil at the receiving terminal is set at the frequency ω0.
However, in the power transmission process, the coupling of the transmitting coil and the receiving coil may change, such as a sudden increase in coupling or a sudden increase in magnetic field energy in the transmitting coil, which will cause DC voltage Vrect to exceed the preset value after full bridge rectifier circuit. The excessive voltage may damage to post-stage DC-DC converter, and even damage to the electronic device on the load side.